A buddy of mine is having some running problems with his 54 chevy truck. The motor is a 350 with an edelbrock carb. The distributer is stock body and cap with electronic ignition module. The advance weights are locked and timing is controlled electronically via an amethyst control unit . The problem is driving us nuts . So the engine will fire on the ****on idles sweet and revs cleanly when either throttled from the carb side or at the gas pedal . The carb always returns to idle . absolutely zero problems with that side of things . The issue arises when the car is in drive and you are pulling away from a stop . If you step on the gas as though to pull away in a sensible manner, instead of the car just pulling away gently, all hell brakes loose , the rear wheels spin and the car takes off like a bat out of hell. If you immediately lift your foot off the throttle the pedal will come back but the car will accelerate for a short distance before it decides of its own accord to idle down . Strangely it will on a lot of these occasions blow the HT lead out of the coil .. If you are really gentle with the throttle you can idle it away from a stop and ones your rolling along generally increase throttle input without a problem , but mash the throttle and the HT lead will blow out the coil again.. I didnt believe it at first but he brought the truck down so I could have a look at it . The carb is in good condition , the accelerator cable is free of any bind and runs straight and isnt under any tension . The pedal isnt binding, took the cable off and checked it was free. The carb lincages are all free and working , Return springs are in situ. Vacuum ports not required are blocked with caps, There was nothing visually wrong with the carb but to eliminate it we put on a direct replacement off the shelf and guess what, it was exactly the same .. So we moved to the ignition side . maybe a faulty coil ? swapped the coil for a known working coil , NO CHANGE. clutching at straws we then replaced the HT lead , wierdly it stopped popping the lead but the mad initial power surge from start was still present .. Checked the vacuum to the gearbox and it was attached. Fuel is delivered via the stock mechanical pump, fuel lines are clear and a new fuel filter fitted . Its like it gets a shot of fuel and just takes off from standing .. yet shows no indication of a fuel delivery issue when not in drive , viz when your just sat in neutral reving it and letting it drp back to idle , it works 100% , We disconected the kick down cable ( TH350) no change, Clutching at straws and lots of head scratching later we are no further forward . I suspected the amethyst advance gizmo but on reflection with zero throttle how could it continue to accelerate for a few seconds ? could timing / excess fuel cause the problem.. Got to be honest,Im kinda stumped by the symptom , never seen anything like it , especially the HT lead being blown off the coil .. we simulated pulling away from a junction and 100% of the tests resulted in initial wheelspin on a light throttle ( not mashing the pedal for deliberate wheelspin) and this short period of continued accelaration when the pedal is released .. It really feels like it continues to gain momentum before it runs out of steam or you brake to rein it in ... Mash the throttle and the lead pops off ... Its got to be so simple were overlooking it .. Hes close to buying a box of matches and increasing his insurance valuation LOL..
I'd chuck the electronic advance **** ,counterweights , springs & vacuum will do the job just fine , very little to be gained " electronically " ( other than gremlins ) !
Stock HEI with std. delco module, std. weights as it came, stock or slightly upgraded coil and timing set at factory spec. or a little more advance. I've found its always better to start with a base (stock) and then build on the base. If a problem appears after the "upgrade"-you know what it is.
I agree with Pontmerc above if you have a solid throttle linkage. Try power braking it. (Hold your foot tight on the brake and drop it into gear, then press on the gas pedal and see what happens. Be ready to hit the ignition switch off, and don' try it in the garage or have someone standing in front of your car.) I had a '62 Ford that did the same exact thing, but only when you made a left turn. The engine would torque up due to the left inertia, and force the linkage wide open. Like it was possessed.
Also check the radiator to see if the fan is contacting he radiator. A broken motor mount will allow this to happen especially under hard acceleration
I got to admit I am stumped, that said, you did a good job at describing the problem. I have always gone with a cable operated throttle on most of my builds and the 84 ford pu cable is simple and doesn't look like ***.
Thanks for all the replies . The motor mount theory is worth investigating , however it does have a flexible throttle cable .. and its a non permanent issue , You dont have to flip the throttle pedal back with your toe. You throttle on as per a normal pull away from a junction , it all goes wild, you take your foot off the pedal, the pedal returns immediately but the motor still pulls like a train for a few yards then settles down.. ... would that sound like a symptom of the motor mount failure ? Torgue converter failure is interesting , would that cause the whoahhhh non deliberate launch ? Wish I had a vidoe of it in action ..I might arrange that .. If you lightly tickle the throttle the take up is OK but it means you would have to creep into traffic from junctions a brisk application of the go pedal will just result in a full on tyre spinning episode and a short length momentary drag race .. Fully agree with the electronics in the bin theory.. Im not a fan , but he is a tech geek I did question what settings he programmed into the set up and he ***ures me its set to stock advance curve . The yehahh launch is bad enough to diagnose but the pop off HT lead is just another little teaser ..
I’d first go to motor mounts, and as described above, don’t stand in front of the car or do it in a space that could be detrimental. If not the engine mount, make sure that the throttle blade screws are still in the throttle shaft. ive had a blade screw fall out on my motorcycle that did the same thing you described. The engine will rev much higher with less throttle in neutral. With a load, you are opening the throttle more for the same rpm reaction. It is possible that with a plate screw missing, and a wider throttle angle, that the plate slips out of place momentarily and holds the throttle open. This is what happened to me.
A bit of fuel ( pump shot) rapidly advancing timing ( electronically controlled timing F U ) = runaway rpm !
Got a picture of how the throttle cable is hooked up to the carburetor? Is the throttle easy to push?
Carb was swapped for a replacement off the shelf that come from a running car with no problems and the same problem existed .. My initial thoughts were timing, especially since this set up is not only fired with a module but has a second gizmo that controls the timing . This unit is programmed via a PC , way over my head . He insists that the timing is set correctly , I have no idea as I don't really get involved in anything more technical that Points and condensers when it comes to ignition systems .. The timing advance did raise a question but that was countered with why would the carb continue to receive fuel when the throttle was closed .. I did theorise that it could be the fuel in the fuel bowl being ****ed in but that was just a theory .. Got to admit I was getting splinters under my nails with the amount of head scratching , but im swiftly returning to the timing being the cause , why else would there be a secondary problem with the HT lead blowing off the coil ( a real bizarre issue)
I checked that , the cable has a good straight run from the carb to the pedal with no bind as the pedal is depressed . It p***es through the firewall via a threaded bulkhead fitting .The pedal was also stripped down to check the bushings for any burrs .Everything was fine , cleaned and rebuilt . The cable was set up with no tension just adjusted so things were light and easy to operate.. Its blooming baffling and more reason why im starting to lean toward the invisible black magic electronic timing gizmo..
With enough flow ( rpm) sufficient fuel can be pulled through the idle & transition circuits , even with the throttle blades closed ,to do what you're experiencing ..IMO
What’s the pedal ratio on the accelerator? Should be 3-4” of foot travel between idle and WOT. Where’s the coil mounted? On engine or firewall How much slack in it? It’s getting air and fuel, so the carb is mostly open more than you thinking it is. Got a go pro to mount under the hood ?
Had a 61 Impala that did about the same thing. Turned out to be bad motor mount. Replaced both mounts and problem disappeared .
Are the engine mounts side mounted or early close together front mounted? I’d be asking myself what was the last thing done to the truck before the problem started?
How tight is the cable? Does it have any slack, or is it a straight line and tight? If there isn't any slack in the cable, a broken mount could allow the engine to raise high enough and allow the cable to pull the carb open. I'd check both ends of the cable housing to make sure they are held tight, not just pushed up into a fitting. Also, if there isn't a swag or even a half loop in the cable housing, I'd try a longer cable and housing to give the motor room to move without pulling the cable. I have a half loop in mine, it retracts and pulls easily.
Engine mounts should not have anything to do with this issue as he has cable linkage. That was the issue with 58-mid 60's V8 cars. On the other hand if the throttle cable was short without any slack in it in a direct run to the carb it could act like a solid linkage in a 62 impala with a broken motor mount. I say 62 because I had a 62 Impala with a 283 do just that when the engine lifted up off the broken mount. If that cable didn't have any slack in it and was stretched tight that could be the problem. The other issue I have run into is springs that aren't strong enough or springs that were hooked up wrong.
These guys have given some good advise and the best I can do is tell you I've had a couple Edelbrocks with secondary drip. The secondary ****erflies weren't closing all the way.
GM had a campaign on the cars with the old mounts 58-69 where you installed a cable with the exhaust manifold bolts around the shaft of the upper control arm. Newer mounts 70-96 cars have a built in stop. With all that said I have seen a broken motor mount do what you describe (except loosing the high tension lead). But these cars had the big cap HEI. Don’t be stubborn check your motor mounts.
Thanks again. Im noting all of the replies and ideas, some I have covered and some need to be examined . I don't have the truck here its at my buddies but when I go up there I have a list of check items to tick off . Ive never came across anything like it before . The other day he dropped in the conversation that prior to this problem occurring his wife was driving the truck and one of the transmission fluid hoses come loose and dumped the trans fluid . The car was rescued and refilled with fluid . However it wasnt driven straight away as there were some other things being attended to . When the car was back on the road , this problem became apparent . The car was rewired by me with a generic aftermarket loom . What spurred the rewire was a break down problem and starting issue . The truck had a combination of original 50s wire and a rats nest of domestic hose wire and all manner of bodgery . After the rewire everything worked , one of the components of the electronically enhanced ignition set up . It took a bit of persuasion but bits were eventually swapped over and the car started and run. It was now that the burst of power problem showed up .. andit happens 100% of the time . The cable attachment to the carb is with a push on ball and socket arrangement. This is a factory spec part that allows the pull to always be straight without any fixed cable bending keeping the cable free to pull straight regardless of the throttle position. Its not over centring, the throttle needs no manual ***istance to go back to rest . After each even, if you pop the hood the throttle is always fully closed with zero obvious visual clues to give and indication as to what caused the problem . Its going to be interesting to eliminate this one .. It will be one step at a time ..
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